1. Field of the Invention
The present invention relates generally to self-assembled semiconductor quantum dot lasers. More particularly, the present invention is directed towards quantum dot vertical cavity surface emitting lasers (QD-VCSELs).
2. Description of Background Art
Vertical cavity surface emitting lasers (VCSELs) are of interest for a variety of applications. Some of the advantages of a conventional VCSEL include surface emission, a nearly round emission pattern, a low threshold current, and the potential for high-yield, low cost manufacturing and packaging.
FIG. 1 illustrates some of the features of a conventional VCSEL 100. A bottom mirror 105 is disposed on a substrate 102. An active region 110 is disposed between the bottom mirror 105 and a top mirror 120. A conventional VCSEL typically includes a quantum well active region for providing optical gain. A quantum well active region typically includes one or more quantum wells capable of providing a comparatively high optical gain. Optical feedback is typically provided by top and bottom distributed bragg reflector (DBR) mirror structures. In a DBR mirror VCSEL, the mirrors typically comprise pairs of alternating high index and low index semiconductor layers, with each layer typically being approximately a quarter wavelength in optical thickness. The active region is typically a high index region approximately an integer number of half wavelengths in thickness having a gain region disposed in its center.
Quantum dot (QD) VCSELs are of potential interest for a variety of applications. Each quantum dot consists of an island of low bandgap material surrounded on all sides by a higher bandgap material. The low bandgap island of each quantum dot is sufficiently small that each dimension (length, width, and height) is smaller than the thermal deBroglie wavelength over operating temperatures of interest. As a consequence, the quantum dot has its energy states quantum confined in three dimensions, resulting in a delta-like density of states (e.g., a high density of states in a finite energy band around each permissible optical transition, analogous to a density of states for atoms).
Quantum dot active regions have a variety of characteristics that make them of interest for VCSELs, such as potential advantages in regards to temperature sensitivity and high-speed modulation. However, there are several technical barriers that have hindered the commercial exploitation of QD-VCSELs.
One barrier to the commercial exploitation of QD-VCSELs is that conventional quantum dot active regions typically have a peak optical gain that is low compared with quantum wells due to the small fill factor of quantum dots. Moreover, the optical gain at the ground state energy level saturates in quantum dots. The optical gain available from a layer of quantum dots is typically about an order of magnitude lower than that which can be achieved from a quantum well. For example, in edge-emitting lasers, the maximum ground state gain that can be achieved from a single layer of quantum dots is typically in the range of about 5 to 10 cmxe2x88x921.
Another barrier to the commercial use of QD-VCSELs is that many commercial applications have demanding operational requirements. For example, some applications, such as ten-gigabit Ethernet (10-GigE) require that the VCSEL operate in an uncooled transceiver over an extended temperature range (e.g., up to about 85xc2x0 C.), operate at a nominal wavelength of about 1310 nanometers (nm), and have sufficient differential gain over all operating conditions to be modulated at the desired data rate. However, since the maximum ground state optical gain decreases with increasing operating temperature this requirement further exacerbates the difficulty of designing a QD-VCSEL having sufficient optical gain to operate within ambient temperature ranges of commercial interest.
What is desired is a QD-VCSEL with improved manufacturability and desirable performance characteristics.
A quantum dot vertical cavity surface emitting laser has a low cavity loss and a correspondingly low threshold gain. To begin with, at least one of the mirrors of the laser cavity is an ultrahigh reflectivity distributed bragg reflector (DBR) mirror with mirror pairs comprised of alternating layers of high refractive index semiconductor and low refractive index oxide.
Doped intracavity contact layers between the DBR mirrors provide current to a quantum dot active region. In a preferred embodiment, the contact layers have a thickness of about a half a wavelength or less to reduce free carrier loss. In one embodiment, about a quarter of a wavelength or less of each contact layer is heavily doped. The heavily doped portions of the contact layer may be positioned to have a low optical overlap with the longitudinal mode to reduce the free carrier loss.
In one embodiment, additional mode control layers are disposed between the DBR mirrors and the active region to reduce the optical overlap of the mode in doped regions and increase the optical confinement in the active region. In a preferred embodiment, the mode control layers are approximately quarter wavelength thick regions, have a refractive index different than adjacent layers, and are positioned to produce resonant reflections that beneficially increase the optical confinement of the longitudinal optical mode in the quantum dot active region and reduce optical confinement in heavily doped contact regions.
In one embodiment, each ultrahigh reflectivity DBR mirror is formed using a lateral oxidation process to convert oxidizable semiconductor layers into low refractive index oxides. In one embodiment, delamination of laterally oxidized mirror layers is inhibited by including intermediate composition layers to reduce residual stress. In another embodiment, one or more openings is arranged to permit lateral oxidation of bottom mirror regions while preserving lateral support regions to support the bottom mirror layers.